15.3.24 Boost Format Strings

Boost format strings are described in the documentation of the
boost::format class, at
http://www.boost.org/libs/format/doc/format.html.
In summary, a directive has either the same syntax as in a C format string,
such as ‘%1$+5d’, or may be surrounded by vertical bars, such as
‘%|1$+5d|’ or ‘%|1$+5|’, or consists of just an argument number
between percent signs, such as ‘%1%’.